create common.mk for os projects

Makefiles of rvos projects contains too many duplicated contents.
Cleanup and move it into a new common.mk file for os only, this will
not touch asm samples.

Finally the common..mk which was used for both asm & os is removed.

Signed-off-by: Wang Chen <wangchen20@iscas.ac.cn>

code/os/03-contextswitch/Makefile

@@ -1,5 +1,3 @@
-include ../../common.mk
-
 SRCS_ASM = \
 	start.S \
 	mem.S \
@@ -12,41 +10,4 @@ SRCS_C = \
 	page.c \
 	sched.c \
 
-OBJS = $(SRCS_ASM:.S=.o)
-OBJS += $(SRCS_C:.c=.o)
-
-.DEFAULT_GOAL := all
-all: os.elf
-
-# start.o must be the first in dependency!
-os.elf: ${OBJS}
-	${CC} ${CFLAGS} -T os.ld -o os.elf $^
-	${OBJCOPY} -O binary os.elf os.bin
-
-%.o : %.c
-	${CC} ${CFLAGS} -c -o $@ $<
-
-%.o : %.S
-	${CC} ${CFLAGS} -c -o $@ $<
-
-run: all
-	@${QEMU} -M ? | grep virt >/dev/null || exit
-	@echo "Press Ctrl-A and then X to exit QEMU"
-	@echo "------------------------------------"
-	@${QEMU} ${QFLAGS} -kernel os.elf
-
-.PHONY : debug
-debug: all
-	@echo "Press Ctrl-C and then input 'quit' to exit GDB and QEMU"
-	@echo "-------------------------------------------------------"
-	@${QEMU} ${QFLAGS} -kernel os.elf -s -S &
-	@${GDB} os.elf -q -x ../gdbinit
-
-.PHONY : code
-code: all
-	@${OBJDUMP} -S os.elf | less
-
-.PHONY : clean
-clean:
-	rm -rf *.o *.bin *.elf
-
+include ../common.mk